Biomedical subjects

W Skalli

Publications and source records attributed to W Skalli.

49 records · Page 3Linked to original sources

PubMed2001

[Quantitative 3D anatomy of the lumbar spine].

PURPOSE OF THE STUDY: There are many anatomic descriptions of the spine, but most concentrate on qualitative information. Quantitative data is however important to achieve a better clinical approach, to adapt implant size and to construct geometric models of spine mechanics. MATERIAL AND METHODS: We examined 32 dry spines (160 lumbar vertebrae) obtained from the Orfila Museum anatomy laboratory at the Saints-Pères School of Medicine in Paris. We drew 178 landmarks on the surface of each vertebra and recorded the 3D spatial coordinates of each point using a Fastrack electromagnetic system operating with +/- 0.2 mm precision. The coordinates of the digitalized points were expressed in a local x-y-z axis field (x=posteroanterior axis, y=right-left axis, z=caudocranial axis). The origin O was half way between the "centers" of the vertebral plates. After calculating 112 linear, angular and surface parameters, results were analyzed with the Statview statistics system. RESULTS: All parameters exhibited gaussian distribution. The transpedicular vertebral depth, corresponding to the maximal penetration of a pedicular screw before touching the anterior wall, was nearly constant: 48 mm (mean). The mean height of the pedicle was approximately 16 mm for L1 to L4 and 21 mm for L5. Pedicle width was 7 mm for L1 and L2 then rapidly widened to reach 10 mm for L5. It was noteworthy that the narrowest pedicle (4 mm) was found in 10% of the L1 vertebrae. There was an exponential rise in the sagittal tilt of the pedicles from L1 to L5, measuring approximately 8 degrees for L1 and rising to 24 degrees for L5. DISCUSSION: Our sample of human spines with unknown clinical characteristics (age, sex) is representative of anatomy laboratory populations, generally composed of subjects over 40 years of age, and is thus adapted for studies of the degenerative spine. Our findings are generally in agreement with data reported in the literature and also provided complementary quantitative data concerning the transpedicular vertebral depth that was found to be a rather constant feature of the lumbar spine. It measured between 40 and 56 mm for 95% of the study population. The dimensions of the pedicle is particularly important: the width must be known to determine the size of pedicle screws; it measured between 7 and 12 mm in 95% of the population. The largest mean cross section of the pedicle was found for L5 (82mm(2)), but measured less than 60 mm(2) in 10% of the vertebrae, suggesting predisposition to spondylolysis.

Anthropometry↗

PubMed2000

3D reconstruction method from biplanar radiography using non-stereocorresponding points and elastic deformable meshes.

Standard 3D reconstruction of bones using stereoradiography is limited by the number of anatomical landmarks visible in more than one projection. The proposed technique enables the 3D reconstruction of additional landmarks that can be identified in only one of the radiographs. The principle of this method is the deformation of an elastic object that respects stereocorresponding and non-stereocorresponding observations available in different projections. This technique is based on the principle that any non-stereocorresponding point belongs to a line joining the X-ray source and the projection of the point in one view. The aim is to determine the 3D position of these points on their line of projection when submitted to geometrical and topological constraints. This technique is used to obtain the 3D geometry of 18 cadaveric upper cervical vertebrae. The reconstructed geometry obtained is compared with direct measurements using a magnetic digitiser. The order of precision determined with the point-to-surface distance between the reconstruction obtained with that technique and reference measurements is about 1 mm, depending on the vertebrae studied. Comparison results indicate that the obtained reconstruction is close to the actual vertebral geometry. This method can therefore be proposed to obtain the 3D geometry of vertebrae.

Cervical Vertebrae↗

PubMed1998

Characterization of the mechanical behaviour parameters of the costo-vertebral joint.

This in vitro study introduces a new method to determine quantitative parameters characterizing the mechanical behaviour of the costo-vertebral joint. These parameters are useful in building numerical models of the thoracic spine, taking into account the thoracic cage. Nine thoracic cages were isolated from fresh human cadavers. From each cage, three functional units were tested: T1-T2, T5-T6, T9-T10. Loads were applied according to the joint local coordinate system. Every functional unit was tested first intact and again after section of successive costo-transverse ligaments. We used an opto-electronic system to follow the three-dimensional motion of the joint, and obtained non-linear load/displacement curves according to the primary rotation axis. A statistical analysis of these curves allowed the calculation of parameters describing the joint mechanical behaviour: total range of motion, motion in the low-stiffness zone, and flexibilities in the positive and negative quasi-linear zones. These values can be used as a database for mechanical modeling of the spine.

Adult↗

PubMed1997

Intraoperative optoelectronic analysis of three-dimensional vertebral displacement after Cotrel-Dubousset rod rotation. A preliminary report.

STUDY DESIGN: This study analyzed intraoperatively the three-dimensional displacement of vertebrae during rotation of the Cotrel-Dubousset rod for scoliosis correction, using an optoelectronic method. OBJECTIVE: To evaluate three-dimensional transitions and rotations of instrumented and uninstrumented vertebrae, produced by the Cotrel-Dubousset instrumentation "derotation" maneuver. SUMMARY OF BACKGROUND DATA: Published reports indicate that Cotrel-Dubousset instrumentation has been more effective in producing spinal derotation than vertebral axial derotation, but no study analyzed intraoperatively the effects on the vertebrae produced solely by rotation of the rod. METHODS: Eight patients with idiopathic scoliosis treated with Cotrel-Dubousset instrumentation underwent intraoperative optoelectronic monitoring using infrared cameras (Vicon). Markers were implanted in the spinous processes of the lower and upper instrumented vertebrae (LIV, UIV), the adjacent uninstrumented vertebrae below and above (-LIV, +UIV), and the apical vertebra. During rod rotation, acquisition and processing of cameras data were performed to obtain three-dimensional displacements of vertebrae. RESULTS: Translations and rotations of LIV and UIV were in identical directions to those of -LIV and +UIV, respectively. Orientation of the LIV hook influenced the displacement of LIV and -LIV. Posterior translation of the apical vertebra was commonly observed in thoracic King II, III, or V curvatures (apical vertebra = T9), and anterior translation in King I and IV and thoracolumbar curvatures (apical vertebra = T11-T12). Axial rotation of the apical vertebra was increased in thoracic curvatures and decreased in thoracolumbar and lumbar curvatures. Lateral translation was the major displacement observed. CONCLUSIONS: Rotation of the rod produces rotational and translational changes along each axis. These results are preliminary, but substantial. Technical improvement would allow more accurate results in the near future.

Adolescent↗

PubMed1997

A three-dimensional parameterized finite element model of the lower cervical spine. Study of the influence of the posterior articular facets.

In this study, we present a three-dimensional geometrical and mechanical finite element model of the complete lower cervical spine. The geometry of the vertebrae is parameterized which allows the model to fit different morphologies of vertebrae. The results obtained with a reduced functional unit model (without posterior arch) and with a complete functional unit model were compared with those obtained from experimental studies, when moments of flexion, extension lateral flexion and axial torque were applied. General agreement was observed. Since the model was parameterized, it was possible to study the influence of some geometrical parameters on the mechanical behavior of the cervical spine. Particularly, we focused on the influence of the posterior articular facets as their geometry is very different from those of the other spinal levels and as large inter-individual variability can be observed. The orientation of the facets with regard to the horizontal plane appeared to have a large influence on the 'coupled rotation to principal rotation' ratio, notably in lateral flexion.

Biomechanical Phenomena↗

PubMed1997

Intervertebral disc prosthesis. Results and prospects for the year 2000.

Presently, the kinematic disc prosthesis model (SB Charité) is the best disc replacement compromise, and is the basis of the evolution of the prosthetic concept at the dawning of the year 2000. Clinical results of a homogeneous series of 105 cases with a mean followup of 51 months show 79% of the patients had an excellent result and 87% returned to work, radiologically, these results correlated with restoration of a well balanced lordosis and with segmental mobility. Factors leading to failure are posterior facet arthritis, osteoporosis, structural deformities, and secondary facet pain. Two- and 3-dimensional numeric modeling enables one to study the total facet joint loading and the maximal local loading on the facet. Dissociation of the stiffness in pure rotation and stiffness in translation of the disc are the bases of the technologic improvement.

Activities of Daily Living↗

PubMed1997

[A protocol of in vivo 3D experimental evaluation of global posture and motion of the spine].

PURPOSE: The aim of his study was to assess 3D global posture and movement of body segments, especially for scoliotic subjects. As scoliosis is a three-dimensional deformity, it needs three-dimensional evaluation and correction, but there is no mean today to get 3D dynamic examination of the whole body. MATERIAL AND METHODS: Using opto-electronic methods, an experimental protocol was established to compare the pre- and post-operative results of treatment. Firstly, the reliability of the protocol was tested in healthy adult subject. Secondly, a reference group of 15 healthy teenagers was analysed Besides, first scoliotic subject in pre- and post-operative situations were followed. The markers fixed on the skin allowed us to calculate the position of the head, the pelvic, the shoulders and the spinal axis, during a static trial and motions. RESULTS: The reliability of the protocol was satisfactory (standard deviation (s) < 5.4 degrees in a flexion movement). The inter-subject variability was greater for the position of the head than for the pelvis, the shoulders or the spinal axis. The scoliotic patient showed a straightening of the whole body in the three anatomic planes. One month after treatment, the range of motion were reduced (+13.8 degrees for the pelvic flexion during a flexion movement), but six months after surgery they were greater than before (+14.7 degrees). CONCLUSION-DISCUSSION: Many systems have been proposed to measure the motion of the trunk, but they were not three-dimensional. The opto-electronic method is a non invasive, external and dynamic system.

Adult↗

PubMed1997

[Computer graphic analysis of the three dimensional deformities of scoliotic vertebrae].

GOAL: A computer graphics method that permits the reconstruction, visualization and measure of the vertebral deformities of the scoliotic spine is presented. MATERIALS: Medical imaging techniques utilizing computerized tomography is at the foundation of the reconstruction technique. The studied morphometric parameters are: 1) vertebral body wedging, 2) transverse and spinous process orientation and dimensions and 3) bilateral variation of pedicular dimensions. RESULTS: The reconstructed specimen showed the usefulness of this technique for visualizing and measuring vertebral deformities. Preliminary results seem to be in agreement with the literature concerning the deformities of scoliotic vertebrae. CONCLUSION: This tool will be useful in morphometric investigations for the evaluation of the deformations of scoliotic vertebrae.

Computer Graphics↗

PubMed1995

Quantification of three-dimensional vertebral rotations in scoliosis: what are the true values?

STUDY DESIGN: The aim of this study is to quantify differences between three-dimensional rotations in space and their calculated values, either on two-dimensional projections (radiographs or computed tomographic scans) or three-dimensional calculations using various mathematical procedures. OBJECTIVE: To use a vertebral model to quantify differences between three-dimensional rotations and their calculated values, using two-dimensional projections or various three-dimensional mathematical procedures. METHODS: A specific program allowed us to move a geometric vertebral model in space using given values and sequences of lateral, sagittal, and axial rotations. Differences in positions due to different sequences were visualized and quantified. Differences due to rotation around global or vertebral axes were considered. RESULTS: For rotations of about 10 degrees, differences are about 2 degrees between three-dimensional and projected angles. Differences increase when combined rotations are large, as generally occurs in a scoliotic spine. They reach 16 degrees for lateral and sagittal rotations of 30 degrees. CONCLUSION: Axial rotation measured on transverse projection is misleading for vertebrae rotated in space. Moreover, dealing with large three-dimensional rotations is meaningful only if the used mathematical convention is given.

Computer Simulation↗

PubMed1995

[Geometrical modeling of the spine and the thorax for the biomechanical analysis of scoliotic deformities using the finite element method].

In order to study the biomechanical behavior of the whole human spine and thorax, as well as orthopaedic treatment effects, a new generation model is proposed, which includes a precise functional representation of the posterior part of the spine, while respecting computational capabilities. This paper presents the geometrical aspects of this model. The latter is built using an hybrid method which combines steroradiographic 3-D reconstructions of the spine and thorax [1] to serial CT scan 3-D reconstructions of typical human vertebrae and sternum [4] and published morphometric data of ribs [2, 3]. These anatomical structures were deformed in order to fit as well as possible the personalized data of scoliotic patients using geometrical transformations as well as interpolation or extrapolation techniques. In the posterior part, articular facets are modelled and parameterized as elementary surface shapes (plane, cylinder, sphere). For the articular facet geometry of a given normal subject, results revealed that the zygapophyseal facets are better represented by planes for T1 to T11 and by portions of cylinders for T12 to L5, which is in concordance with the literature [5, 6]. Evaluation of this modelling approach was done on 2 cadaveric vertebral segments. Parametric data obtained from the model were compared to precise measurements done on the vertebrae using a 3-D digitizer, and concordance was found. These personalized geometric informations were then used to build a finite element model [7], which will be useful to study scoliotic deformities as well as personalized orthopaedic treatments.

Humans↗

PubMed1994

Influence of geometrical factors on the behavior of lumbar spine segments: a finite element analysis.

The main objective of this study was the assessment of the influence of geometrical factors on the behavior of lumbar segments. To this end, a three-dimensional, parameterized, finite element model of the lumbar spine was used, and the results were compared with inhouse experimental results and with the few published experimental results available concerning either the geometry of the tested samples or the differences observed at different vertebral levels. Furthermore, in order to appreciate the relative importance of the geometry, the influence of the variation of some other parameters was studied, such as the orientation of the facet joints, the gap between the articular processes, and the Young's modulus of the disk fibers. As a first approach, a series of computations was carried out in order to evaluate the role of geometry in the mechanical behavior differences observed at different levels. It has been found that geometrical factors do exert a noticeable influence on the behavior of the spine, especially those which interfere with the dimensions of the intervertebral disk.

Biomechanical Phenomena↗

PubMed1993

A biomechanical analysis of short segment spinal fixation using a three-dimensional geometric and mechanical model.

Vertebral stabilization using spinal fixation devices is a widely used technique. A three-dimensional geometric and mechanical finite element model has been used as a simulation tool for the evaluation of the mechanical behavior of spinal devices. The geometry of lumbar vertebrae was parameterized, which allows the construction of the geometric model for a given lumbar segment from the digitization of two roentgenographs. This procedure was used to construct a finite element model for a three-vertebra segment with simulation of fractures in the middle vertebra, and with simulation of a restoration using an osteosynthesis device, implemented in a frame fashion with four screws and two rods linked by two transverse rods, and/or an anterior bone graft. Compression force and torsion moment were considered, and different cases were investigated, by varying the severity of the fracture, the geometric characteristics of the device, and the mechanical characteristics of the material joining the two intact vertebral bodies. Results were analyzed considering the mobility of the vertebral segment, which indicates the ability of the restoration system to stabilize the vertebral segment, and considering the forces and moments distribution in the device, which gives information on part of the forces that pass through the device in each situation. Results show that maximum values of forces and moments in the device are more important in compression than in torsion. Adding an anterior bone graft has an effect mainly for compression, whereas in torsion its effect is negligible. For a rigid fixation device, no significant difference was found between different fracture models, indicating that the posterior arch does not play an important role for an instrumented segment. For compression, a rigid posterior wall, or the presence of a bone graft, reduces greatly the mobility of the instrumented segment. For torsion, suppressing the two transverse rods in the device greatly increases the mobility of the instrumented segment. Using a finite element model of a lumbar vertebral segment appears to be an interesting tool to analyze the behavior of an instrumented spine and to compare between different stabilization systems.

Biomechanical Phenomena↗

PubMed1992

Three-dimensional geometrical and mechanical modelling of the lumbar spine.

The main objective of this study is to design a three-dimensional geometrical and mechanical finite element model of the lumbar spine. The model's geometry is constructed using six parameters per vertebra. These parameters are digitized from two X-rays (anterio-posterior and lateral), thus yielding an individualized model which can be arrived at from the radiographs of a tested specimen. This procedure makes the model validation easier, as geometry is generally a factor of dispersion in experimental results. The geometrical reconstruction, in the form of a finite elements mesh, was effected for the whole lumbar spine. The global coherence of the model was verified.

Humans↗